Tantalum metal is usually produced by reduction of potassium tantalum fluoride (K.sub.2 TaF.sub.7) with metallic sodium. Potassium tantalum fluoride is therefore an important intermediate in the production of tantalum metal.
Production of the metal starts with tantalum ores or concentrates, which are dissolved in hydrofluoric acid; the tantalum goes into solution as the fluoride. The tantalum fluoride is extracted from the aqueous hydrofluoric acid solution in known manner by liquid-liquid extraction employing an organic extractant, after which the organic phase is separated. The tantalum is obtained from the organic phase in a form suitable for processing (i.e., as potassium tantalum fluoride) by precipitation with potassium fluoride; it is precipitated from organic solution. The purity of the potassium tantalum fluoride thus obtained is dependent essentially on the purity of the chemicals used.
If one prepares for example potassium tantalum fluoride according to German Pat. No. 1,018,037, wherein it is precipitated from solution in an organic solvent, traces of the latter remain on the potassium tantalum fluoride, so that the potassium tantalum fluoride must be recrystallized to effect further purification. Silicon impurities, however, are not substantially removed by such recrystallization.
Contamination by the organic solvent can be avoided according to U.S. Pat. No. 3,403,983 by steam distillation of the organic solution before precipitation. After the distillation, an aqueous solution of tantalum fluoride remains, from which the potassium tantalum fluoride can be crystallized by adding a potassium salt. This process is costly in terms of equipment and the amount of energy consumed.
The foregoing methods have the additional disadvantage that the silicon contained, as an impurity, in the chemicals used for processing, is concentrated in the potassium tantalum fluoride. But potassium tantalum fluoride containing more than 15 p.p.m. by weight of silicon as Si is generally commercially unacceptable.
The known methods require therefore that chemicals with an extremely low silicon content be used for the preparation of usable potassium tantalum fluoride. The preparation of ammonium fluoride, potassium fluoride, potassium hydrogen fluoride, and hydrofluoric acid with extremely low silicon values is so costly, however, as to make their use commercially prohibitive.